The present invention relates to digital communications, and more particularly, to method and apparatus for combating impulse noise in digital communications channels.
Impulse noise is typically characterized by a high power level during a short time period and a significantly lower power level during the rest of the time. Impulse noise is a major problem in several digital communications channels including high-speed CATV (cable TV) channels and high speed DSL channels. For example, in return CATV channels, impulse noise is considered as the primary channel impairment (xe2x80x9cResults of Return Plant Testingxe2x80x9d, R. Prodan, 1997 National Cable Television Association (NCTA) Technical Papers). In certain CATV plants, noise impulses occur at a rate of thousands of impulses per second, with a typical duration of less than a micro-second, but the noise power during the impulses is higher than the power of the signal.
There are several approaches to combat impulse noise in digital communications channels (see, xe2x80x9cError Control Coding, Fundamentals and Applicationsxe2x80x9d, Shu Lin and Daniel J. Costello, Jr., Prentice Hall, Inc. Englewood Cliffs, N.J. 1983; xe2x80x9cDigital Communicationsxe2x80x9d, John G. Proakis, third edition, McGraw Hill 1995):
1. Forward Error Correction (FEC) coding (for example, Reed-Solomon FEC). The disadvantage of this approach is that it requires spending a portion of the transmitted data for redundancy bits, thus reducing the information rate of the transmitter.
2. Using low symbol-rate signaling. When the symbol period (which is equal to the inverse of the symbol-rate) is much longer than the impulse duration, a conventional receiver reduces the effect of a noise impulse by applying a conventional matched filter. A drawback of this approach is that it degrades the data-rate attainable by a transmitter-receiver pair. It is possible to replace a high symbol-rate channel by multiple low symbol-rate channels, using a multi-tone approach; the overall data rate is not degraded, but this multi-tone approach will increase the complexity of the transmitter and receiver. A multi tone approach with low symbol-rate signaling is more robust than high symbol rate signaling for low-magnitude short-duration pulses, but less robust for high-magnitude short-duration pulses. With a lower symbol rate, the number of errors per second will be smaller than with higher symbol rate. However, the total number of symbols is also smaller, and hence the average error rate may stay unchanged.
3. Interleaving. The transmitter can interleave the transmitted FEC-encoded symbols over the time axis. This can improve robustness to noise impulses with durations longer than the duration of a FEC symbol (e.g. a character of Reed-Solomon FEC). But in a channel having frequent noise impulses at random times with an impulse duration significantly shorter than a FEC symbol, such an approach will have a marginal effect on the maximum rate of noise impulses that the system can tolerate. Another interleaving approach is calculating digital samples of the modulated waveform at a sampling rate much higher than the symbol rate, interleaving these data samples over the time axis, transmitting the interleaved samples, and performing a corresponding de-interleaving operation in the receiver. Such an approach can significantly improve robustness to impulse noise, as it spreads each impulse over multiple symbols at the de-interleaver output. However, such an approach may cause error propagation in case of a very high magnitude impulse; it also spreads the spectral density of the transmitted signal and therefore is not suitable to bandlimited modulations, such as QAM and PAM. It is more suitable to direct sequence code division multiple access (CDMA) approaches.
The present invention may be used in combination with other methods described herein before for combating impulse noise, resulting in an improved and higher robustness.
This invention is particularly useful as a system for digital communications over channels that suffer from impulse noise, and particularly over return cable TV channels, downstream cable TV channels, and digital subscriber loops (DSL) channels for high speed communications over twisted pair copper lines.
In one aspect, the present invention provides an apparatus for digital communication in noisy communications channels including a transmitter with a pre-filter designed to create deliberate inter-symbol interference so that a plurality of symbol intervals are included in the transmitted signal.
In another aspect, the present invention provides a method for digital communication in a noisy communications channel by creating inter-symbol interference in a transmitted signal and using a multiple symbol time interval in the received signal to reduce impulse noise.
In another aspect, the present invention provides a method for digital communication in a noisy communications channel by creating inter-symbol interference in a transmittal signal and using a multiple symbol time interval in the received signal to reduce impulse noise.
In another aspect, the present invention provides a digital transmitter having a forward error correction encoder for receiving an input data stream and providing an encoded output data stream, a filter having an impulse response with unity normalization and multiple taps with high magnitude for receiving and filtering said output data stream, and a modulator for modulating said filtered output data stream.
In another aspect, the present invention provides a digital transmitter having a forward error correction encoder for receiving an input data stream and providing an encoded output data stream, a filter for deliberately injecting inter-symbol interference into said output data stream, and a modulator for modulating said filtered output data stream.
In another aspect, the present invention provides a digital receiver having a demodulator for receiving an input data signal, down converting said signal, and sampling said signal to provide an input data stream, a smoother for receiving said input data stream, identifying data samples that are likely contaminated by impulse noise, and attenuating data samples likely contaminated by impulse noise, a filter and timing recovery portion for applying a square-root raised cosine filter to data samples output by said smoother and for determining the sample rate and sample clock phase, a filter for filtering the output from said filter and timing recovery portion having an impulse response sequence with a small peak to average ratio that attenuates the effect of impulse noise with a duration less than the duration of a symbol, an adaptive equalizer for receiving the output of said filter and compensating for carrier phase, undesired inter-symbol interference, and stationary noise components, a modified maximum likelihood sequence estimation (MLSE) decoder for applying a viterbi MLSE using a plurality of non-zero taps to determine a sequence of symbols in a data stream received from said adaptive equalizer, and a forward error correction (FEC) decoder that utilizes indications of samples contaminated with noise from said smoother to perform error correction on said data stream. In another aspect, the present invention provides a method for detecting a noise impulse in a data stream by associating an indication with a data sample, comparing the absolute value of the amplitude of said data sample with a first preselected threshold value, setting said indicator to a first predetermined value if said amplitude is greater than said first preselected threshold value, comparing said amplitude to a second preselected threshold value if said amplitude is less than said first preselected threshold value, setting said indicator to a second predetermined value if said amplitude is greater than said second preselected threshold value, and setting said indicator to a third predetermined value if said amplitude is less than said second preselected threshold value.
In another aspect, the present invention provides a method for attenuating a noise impulse in a data stream by determining a residual error value for a data sample, comparing the absolute value of the amplitude of said residual error value with a first preselected threshold value, modifying the amplitude of said data sample to a first predetermined value if said amplitude is greater than said first preselected threshold value, comparing said amplitude to a second preselected threshold value if said amplitude is less than said first preselected threshold value, and modifying the amplitude of said data sample to a second predetermined value if said amplitude is greater than said second preselected threshold value.
In another aspect, the present invention provides an apparatus for digital communication in noisy communications channels including a transmitter including a pre-filter for creating deliberate inter-symbol interference such that a plurality of symbol intervals are included in a transmitted signal, and a receiver having a detector constructed to combat impulse noise by using multiple symbol time interval diversity.